Metamaterial slows light for quantum computing

In quantum computing, light pulses are used to manipulate, read, and exchange information about the state of quantum bits, or qubits. Controlling the speed of light thus controls the flow of information in these systems. Stopping the light could enable quantum memory.

A metamaterial that can completely stop light would store the light’s information in qubits, and releasing the light on demand would read out the memory. Such a metamaterial requires an ensemble of nearly identical qubits. In the optical domain, qubits are made of atoms that are identical by nature. But in the microwave, qubits are artificially made, superconducting, two-level systems, and fabricating a large, identical array of them is difficult.

Brehm et al. developed an on-chip microwave waveguide with several superconducting qubits to controllably slow the speed of light by a factor of 1500. Two factors are important in determining this value: the actual retardation of light and how strongly the light is damped as it propagates through the metamaterial.

“We wanted to demonstrate quantum metamaterial properties with a reasonably sized array of frequency-tunable superconducting qubits,” said author Jan Brehm. “This sets the foundation for ultimately realizing a quantum memory in the microwave domain. Similar experiments were so far mainly conducted with atoms in the optical regime.”

The densely spaced qubit array offers broad potential for quantum optics and solid-state physics, particularly the study of polaritonic excitations and quantum light.

“Next steps will be, for example, studying quantum properties of non-classical light propagating through such a metamaterial, exploring qubit arrays in waveguides with tailored dispersion and bandgaps, and investigating many-body quantum simulators,” said author Alexey Ustinov.

Source: “Slowing down light in a qubit metamaterial,” by Jan David Brehm, Richard Gebauer, Alexander Stehli, Alexander N. Poddubny, Oliver Sander, Hannes Rotzinger, and Alexey V. Ustinov, Applied Physics Letters (2022). The article can be accessed at https://doi.org/10.1063/5.0122003 .